Method for searching a control channel in a mobile station

ABSTRACT

A control channel is searched for call waiting in a mobile station where the power supply for a receiving circuit is turned off during a break time of receiving level measurements, wherein in a first control channel selection mode, all the control channels are divided into a plurality of groups, the receiving levels of the control channels are measured group by group intermittently and sequentially. The intermittent measurement of these groups are repeated until a control channel having a receiving level equal to or greater than a first predetermined value is detected. When a receiving level equal to or greater than the first predetermined value is detected, the receiving levels of all the control channels are measured in a second control channel selection mode.

This application is a divisional of U.S. patent application Ser. No.08/558,286, filed Nov. 13, 1995.

The present invention relates to a method for searching a controlchannel in a mobile station for call waiting or cell switching, andparticularly to such a search method which allows reduction of powerconsumption in the mobile station.

In a mobile communication system, as shown in FIG. 1, a wide servicearea SA is divided into many cells (sometimes referred to as zones) CE1,CE2, . . . . Mobile stations MS (car telephones, portable telephonesetc.) in each cell can be connected to an exchange office (not shown)via a base station BS in the cell. The size of each cell and atransmission power of the associated base station are determined so thatreceiving levels of control channels transmitted from the base stationsof adjacent cells are substantially equal to each other on the border ofthe adjacent cells. A control channel and a speech channel are assignedto each cell and the frequencies of the channels are different betweenthe adjacent cells.

Information (control channel information) for identifying a base stationin a cell and for identifying the base stations in the peripheral cellsis always transmitted from the base station of the cell via a controlchannel allocated to the cell. The control channel is also used to sendand receive information for processing the location registration of amobile station, call-in and call-out. A communication channel is used bya mobile station for speech conversation. One or a plurality of speechchannels are allocated to each cell dependent on the traffic volume ofeach cell. Since the present invention particularly relates to thecontrol channel, only the control channel is explained hereinafter. Amobile station in each cell measures the receiving levels of the callwaiting control channel of the cell and of the control channels of theperipheral cells. When the receiving level of the call waiting controlchannel becomes a predetermined level or more lower than the highest oneof the receiving levels of the control channels of the peripheral cells,it is decided that the mobile station is moving into another cell, and aprocessing for call-waiting is performed with the control channel of thehighest receiving level.

Since the frequency band allocated to the entire communication system islimited, the total number N of the control channels obtainable atintervals of a constant band width .increment.F is limited. Sincedifferent frequencies cannot be allocated to all the cells, the same setof channels, for example CH1-CHM, are repeatedly allocated to differentgroups of the cells sufficiently far from one another so that theinterference can be ignored. Further, in order to efficiently utilizethe frequency band allocated to mobile communications, control channelsCH1'-CHM' shown in row B of FIG. 2 whose frequencies are displaced by afrequency smaller than the control channel interval (band width).increment.F, e.g. .increment.F/2, respectively from the frequencies ofthe control channels CH1-CHM shown in row A of FIG. 2 in a interleavingmanner may be allocated to the cells such that radio-waves of controlchannels whose frequencies are displaced by .increment.F/2 do notinterfere each other. Therefore, if M'=M, a maximum of 2M controlchannels can be used in the entire service area and the receivingcircuit of each mobile station is constructed to be capable of selectingany one of these control channels for the communication.

The number of total control channels is defined as N hereinafter. For amobile station in any cell, the control channel being transmitted by abase station of the cell in which the mobile station is located is thecall waiting control channel in that cell and a control channel whosefrequency is displaced by .increment.F/2 from the call waiting channelis unnecessary channel. Therefore, in an arbitrary cell where a mobilestation is located, a control channel of another cell whose frequency isdisplaced by .increment.F/2 from the control channel being transmittedby the base station of the arbitrary cell is referred to as aninterleave channel hereinafter.

After the power is turned on in a mobile station, in order to be readyfor a call-in and a call-out, it is necessary to find out a controlchannel being transmitted by a base station of a cell where the mobilestation is located and to be in the receiving state (call waiting state)of that control channel. It is also necessary to perform procedures forentering the call waiting state when a mobile station gets out of atunnel or a mountain cove, where a control channel cannot be received,after a long stay therein or when enters the service area from theoutside area. In such a case, a mobile station selects an appropriatecontrol channel from all the control channels and waits for the receiptof a control signal related to the communication control from theselected control channel.

In the conventional system, when a mobile station is powered on and thefirst call waiting process is performed, or when a mobile station getsout of incapable state of signal reception and the first call waitingprocess is performed, channels whose receiving levels are greater than apredetermined level defined by the system are selected as candidates anda decision on whether call waiting possible or not is performed startingfrom the highest receiving level channel in the descending order of thereceiving level for all the candidate channels. That is, first, themobile station selects a control channel candidate of the highestreceiving level and tries to receive the control signal. If the controlsignal can normally be received and call waiting is deemed to bepossible as a result of analysis of the contents of the received controlsignals, a call waiting is performed with the control channel. However,if the control signal cannot normally be received over the selectedcontrol channel, or if the analysis of the control signal indicates thatthe control channel cannot be used even though the control signal cannormally be received, a decision on propriety for call waiting is triedwith the next highest receiving level control channel (next candidate).The conditions by which call waiting is decided to be impossible are thecases where the signal received over the control channel indicates "outof service" due to under construction or under testing, or the receivedsignal is not the control signal used in the communication system bywhich the mobile station is served, or the level of the received controlsignal is below the predetermined level, etc.

The aforementioned decision on propriety for call waiting is performeduntil a control channel by which call waiting is possible is found inthe candidate control channels. If all the candidate control channelsare decided to be unable for call waiting, the receiving levels of allthe control channels are measured again to obtain the candidate controlchannels.

As mentioned above, in a mobile station, since the receiving levelmeasurements are repeatedly performed until a control channel whosereceiving level is equal to or greater than a predetermined value isfound, the receiving levels of all the control channels are measuredrepeatedly and continuously even when the mobile station is locatedoutside of the service area SA. Thus, a problem of an early batteryexhaustion does exist in a portable mobile station etc..

In a mobile communication system where interleave channels are provided,as mentioned previously with reference to FIG. 2, with respect to eachcontrol channel there are two adjacent interleave control channelshaving frequencies within an interval (e.g., interval of .increment.F/2)less than the control channel frequency band width .increment.F, and thefrequency band of each control channel party overlaps such twointerleave control channels. Assuming that such two interleave controlchannels whose frequency bands are adjacent to a control channel CHa(row A of FIG. 2) are CHb and CHc (row B of FIG. 2), if the receivinglevel in the control channel CHa is equal to or greater than apredetermined value in a cell, it is possible that the receiving levelsin the adjacent control channels CHb and CHc may also be equal to orgreater than the predetermined value because of the partial band overlapwith the control channel CHa.

In a cell CEa (FIG. 1), for example, there may be a case when a mobilestation MS which is in the call waiting state with the control channelCHa enters a cove of buildings or a tunnel and is temporarily unable toreceive the control signals, and then the mobile station selects theinterleave channels CHb and CHc as the candidate control channels inorder to search the call waiting channel again and decides propriety forcall waiting. In addition, when the control signal received by themobile station MS over the control channel CHa which is selected forcall waiting in the cell CEa is the signal indicating "underconstruction" or "under test" of the base station and thus the controlchannel cannot be used, similar decision on propriety for call waitingmay be performed on the interleave channel CHb or CHc as an alternativecandidate control channel for CHa. In such a case, the receiving levelin the candidate control channel CHb or CHc selected by the receivingcircuit of the mobile station may not be the receiving level of thecontrol signal from the base station of other cell CEb or CEc where thecontrol channel CHb or CHc is allocated but may be the receiving levelof the control signal of the control channel CHa whose frequency band ispartially overlapped with that of CHb or CHc.

In this case, even if the receiving level is sufficient to satisfy thepredetermined level for call waiting, the decision of propriety for callwaiting always fails because the control signal cannot normally bereceived and the contents cannot be analyzed. Since it usually takesseveral seconds to judge propriety for call waiting for one controlchannel, if there are many candidate control channels which needdecisions for call waiting, it takes a long time to determine the callwaiting channel and a large power is consumed.

As described above, a mobile station possibly performs decision ofpropriety for call waiting on the interleave channels CHb and CHc whichare unable to be call waiting and thus, it takes time to detect controlchannels in which the mobile station can be in call waiting state. Inaddition, when the mobile station is a portable mobile station, thebattery consumption is accelerated.

Once a mobile station enters call waiting state, the mobile stationwaits for a control signal such as a call-in signal from the basestation over the control channel in the cell where the mobile station islocated and also the mobile station always measures the receiving levelsof the control channels of the peripheral cells surrounding the cellwhere the mobile station is located and the receiving level of the callwaiting control channel continuously and repeatedly based on theidentification information of peripheral cells' control channelscontained in the received control signal. For example, as shown in FIG.10A to be mentioned later, when a series of receiving levels of the callwaiting control channel and the control channels of the peripheral cellsare measured in each measuring time T_(M), the measurement of receivinglevels of these control channels is immediately repeated again. For eachconstant time period T_(S) (or constant number of repetitions), anaverage value of the measured receiving levels in the constant timeperiod T_(S) is obtained for each channel. The average receiving levelof the call waiting control channel is compared with the averagereceiving level of each peripheral cell's control channel. If any one ofthe latter is equal to or greater than the sum of the call waitingchannel's level and a predetermined value .increment.L, the processshifts to the procedures for switching to the cell corresponding to thehighest level control channel of those peripheral control channels. Thatis, for each time period T_(S), the process of the averaging thereceiving levels is performed for each of the control channels and thusobtained average receiving levels are compared to decide whetherprocessing for cell switching is necessary.

In the conventional system, regardless of the fading period of thereceiving level or the moving speed of a mobile station, the receivinglevels of the call waiting control channel and the peripheral cells'control channels are measured in a constant period T_(P). When themeasurement of all the receiving levels of the control channels to bemeasured is completed, it is usual to repeat, immediately, themeasurement from the starting control channel. That is, the measurementrepetition period T_(P) is equal to the measuring time T_(M) requiredfor measuring, once, all the control channels to be measured. When themoving speed of the mobile station is high, rapid change in thereceiving levels of the call-waiting channel and peripheral cellchannels are likely to occur and, therefore, it is necessary to make themeasuring period T_(P) and call-switching decision period T_(S) as shortas possible. On the other hand, despite that rapid change might hardlyoccur in the receiving level when the moving speed is low, it has beenconventional to repeat the receiving level measurement and thecell-switching decision with the same short periods T_(P) and T_(S) asthose in the case of the high moving speed mentioned above, thusconsuming undesirably large power.

In addition, in the conventional system, since the period T_(S) fordeciding the cell switching is constant, the cell switching decisionsometimes has to be made based on unreliable measurement results causedby temporary reduction in receiving level in a cove of buildings etc.Hence, it has been frequent to perform processing for the cellswitching, in vain. Therefore, much useless power has been consumed.

SUMMARY OF THE INVENTION

It is a first object of the present invention to provide a method forsearching, with a less power consumption, control channels which can becandidates for a call-waiting control channel.

It is a second object of the present invention to provide a method forsearching, with less power consumption, a call-waiting control channelamong the candidates for call-waiting control channels.

It is a third object of the present invention to provide a method forsearching, with a less power consumption, a control channel for a cellswitching decision during call waiting time in a mobile station.

According to a first aspect of the present invention, when a mobilestation selects candidate control channels for call waiting, in a firstcontrol channel selection mode, a plurality of control channels whichare part of all the control channels stored in a mobile station aresequentially and intermittently measured until a control channel whosereceiving level is equal to or greater than a predetermined value isdetected. As the result, if any control channel whose receiving level isequal to or greater than the predetermined value is detected, then in asecond control mode selection mode, the receiving levels of all thecontrol channels stored in the mobile station are measured to select anoptimum control channel in the cell. If the optimum control channelcannot be selected in the second control channel selection mode, thefirst control channel selection mode is performed again.

In the aforementioned first aspect of the present invention, in thefirst control channel selection mode, a plurality of control channelsare divided into a plurality of groups, receiving levels of the controlchannels are measured group by group intermittently and sequentially.

According to a second aspect of the present invention, in the method forselecting a control channel for call waiting, when the frequencyinterval between arbitrary two selected control channels is less than achannel frequency interval and the receiving levels of these controlchannels are equal to or greater than a predetermined value, only thecontrol channel whose receiving level is higher is defined as thesubject of selection for a call waiting control channel and the controlchannel whose receiving level is lower is excluded from the subject ofselection for a call waiting control channel.

According to a third aspect of the present invention, the fadingfrequency of the receiving level of a call waiting control channel orthe moving speed of a mobile station is detected. If the detected fadingfrequency or the moving speed is less than a predetermined value, thereceiving level measurement repetition period is made longer than thecase where the detected value is equal to or greater than thepredetermined value.

In the above third aspect of the present invention, if the fadingfrequency is lower than the predetermined value, the repetition periodof the cell switching decision is also made longer than the case wherethe fading frequency is equal to or higher than the predetermined value.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows an outline of the service area in the conventional mobilecommunication system;

FIG. 2 shows a frequency arrangement of all the control channels in aservice area;

FIG. 3 is a block diagram showing a basic configuration of a mobilestation where the method in accordance with the first and second aspectsof the present invention is applied;

FIG. 4 is a flow chart for implementing the control channel searchingmethod in accordance with the first aspect of the present invention;

FIG. 5A is an operation timing chart in the conventional control channelsearching method;

FIG. 5B is an operation timing chart in the method of FIG. 4;

FIG. 6 is a flow chart for implementing the control channel searchingmethod in accordance with the second aspect of the present invention;

FIG. 7A is a first table TB1 generated in the method of FIG. 6;

FIG. 7B is a second table TB2 generated in the method of FIG. 6;

FIG. 7C is a table TB2' created by setting the receiving levels of theinterleave channels to zero in the table TB2 of FIG. 7B;

FIG. 7D is a third table TB3 created by deleting channels whosereceiving levels are zero from the table TB2' of FIG. 7C;

FIG. 8 is a block diagram showing the process and the data storage partrequired in the present invention;

FIG. 9 is a flow chart for implementing the control channel searchingmethod in accordance with the third aspect of the present invention;

FIG. 10A shows a receiving level measuring period and a cell switchingdecision period in a high speed moving state in the present inventionand also shows a receiving level measuring period and a cell switchingdecision period in the conventional system;

FIG. 10B shows a receiving level measuring period and a cell switchingdecision period in a low speed moving state in the present invention;

FIG. 10C shows a receiving level measuring period and a cell switchingdecision period in a low speed moving state in the present invention;and

FIG. 11 is a flow chart showing another example for implementing acontrol channel searching method in accordance with the third aspect ofthe present invention.

DETAIL DESCRIPTION OF PREFERRED EMBODIMENTS

The embodiments of the present invention will be explained belowreferring to the drawings.

FIG. 3 shows a basic configuration of a mobile station for implementingthe method in accordance with the first and second aspects of thepresent invention which is to be explained below. A mobile station has areceiving circuit 11, a receiving level memory 12, a control part 13, apower supply part 14 and a transmission circuit 15. The control part 13has a channel memory 13 internally and the frequencies corresponding toall the control channels usable in the service area are initially storedin the channel memory 13. The receiving circuit 11 and the transmissioncircuit 15 are set to a desired channel by setting a frequency read outfrom the channel memory 13 to the receiving circuit 11 and thetransmission circuit 15. The receiving circuit 11 gets into receivingstate in the set control channel, measures the level R of the controlsignal being received, writes the level R into the receiving levelmemory 12 and gives the received control signal S to the control part13. The control part 13 compares the receiving levels of the controlchannels read out from the receiving level memory 12 as required with afirst predetermined value L₁ to select candidate control channels and todecide if the selected candidate control channels are equal to orgreater than a second predetermined value L₂. Also, the control part 13decides if the received control signal S is a proper control signal and,based on these decision results, further decides if any one of theselected candidate control channels is the right control channel forcall waiting the control part 13 further controls the power supply part14 to turn off the power to the receiving circuit 11 while themeasurement of receiving levels is not performed and to turn off thepower to the transmission circuit 15 while transmission is notperformed.

Embodiment According To The First Aspect

For example, when a mobile station is located outside of the servicearea, if the receiving level of the control channel is less than thepredetermined level, signals cannot be received normally. Therefore,continuous monitoring of all the control channels for searching a callwaiting control channel is meaningless. On the other hand, when themobile station is close to the service area, it is desired that a callwaiting control channel can be found immediately. Therefore, in theembodiment of the first aspect, after detecting that the mobile stationis close to the service area in the first control channel selection modeSM1, the call waiting control channel searching is performed in thesecond control channel selection mode SM2 in the same manner as theconventional system.

Even in the case when interleave channels are not used, the skirts ofadjacent frequency bands of the control channels are overlapped as shownin row A of FIG. 2. For example, when a receiving level of either of thecontrol channels (e.g., CH1 or CH3) adjacent to a control channel (e.g.,CH2) of a certain cell is measured in that cell, a leaked power from thecontrol channel CH2 is detected. Therefore, even if the receiving levelsof all the control channels which have been installed in the mobilestation are not measured, the signals of any one of the control channelsfrom the service area can possibly be detected and it can possibly bedetected that the mobile station is approaching to the service area. Forexample, in row A of FIG. 2, if the total number of control channels is9 and those channels are CH1, CH2 . . . CH9 in ascending order offrequency, it can be judged whether or not there is a control channelsignal wave in any one of the channels CH1-CH9 by measuring thereceiving levels of only three channels CH2, CH5 and CH8. From thisreason, it is highly probable that the receiving level equal to orgreater than the predetermined value L₁ can be detected in either casewhere a control channel of a cell in the approached service area matchesthe control channel which the mobile station selects or is a adjacentcontrol channel of the control channel which the mobile station selects.

According to the first aspect of the invention, at least one group ofplurality of control channels selected from all the control channelswhich a mobile station can set are predetermined. The measurements ofreceiving levels on the groups of control channels are repeatedlyperformed intermittently. When a receiving level equal to or greaterthan the predetermined value L₁ is detected in any of the controlchannels, it is judged that the mobile station is close to the servicearea. The power consumption in the receiving circuit can be reduced bysuch intermittent operations.

FIG. 4 shows a flow chart for implementing the control channel searchingmethod in accordance with the first aspect of the present invention.According to the embodiment of the first aspect, in the first controlchannel selection mode SM1, all the, e.g. N, control channels CH1, CH2 .. . CHN pre-stored in the channel memory 13M of a mobile station aredivided into a plurality of, e.g. G, groups. Each of the groups areintermittently and sequentially selected to measure the receivinglevels. Dependent on the received level (whether or not the receivedlevel is equal to or greater than the predetermined value L₁), it isdetermined that the mobile station is approaching to the service area,i.e., to any one of the cells. Therefore, even if a control channelhaving a receiving level equal to or greater than the predeterminedvalue L₁ is not detected in one group, it is highly possible that such acontrol channel can be detected in the other groups.

Incidentally, an arbitrary grouping is possible, for example, as shownin following patterns 1, 2 and 3. Here, for explanation purposes, a casewhere the total number of control channels is 50 and the number ofgroups is 10 (G=10) is shown.

Pattern 1

First group =CH1, CH2, . . . , CH5

Second group =CH6, CH7, . . . , CH10

:

Tenth group =CH46, CH47, . . . , CH50

Pattern 2

First group =CH1, CH1l, . . . , CH41

Second group =CH2, CH12, . . . , CH42

:

Tenth group =CH10, CH20, . . . , CH50

Pattern 3

Channels are randomly allocated to First group--Tenth group.

Incidentally, a method other than the above patterns 1-3 is of coursepossible.

The grouping will be explained in accordance with the sequence shown inFIG. 4. In FIG. 4, G is the number of groups, g is the group number (g=1through G), N is the number of control channels stored in a mobilestation and n is the number of control channels per group (n=N/G).

In FIG. 4, a variable g is set to 1 first in order to specify a group(step S1). Then in step S2, n control channels are selected in the gthgroup (first group) specified by this variable g and the receivinglevels of those channels are measured (step S2). In the above pattern 1,the receiving levels of 5 waves of the control channels CH1, CH2, . . ., CH5 are measured.

As the result of this receiving level measurement, it is decided whetheror not the control channel having receiving level equal to or greaterthan the first predetermined value L₁ does exist (step S3). The firstpredetermined value L₁ is set to relatively small value (e.g., 4-7 dBμ)so that it would be possible to detect, in a control channel, a leakagepower from the adjacent rightous call-waiting control channel to therebydecide that the mobile station is located inside the service area. Asthe result of this judgement, if any control channel having a receivinglevel equal to or greater than the first predetermined value L₁ is notdetected, the process proceeds to step S4 to turn off the power of thereceiving circuit 11 for a predetermined time period T_(OFF). After thepredetermined time period T_(OFF) has passed, the variable g isincremented by one in step S5 and then g is checked to see if g=G+1. Ifg is not G+1, the process returns to step S2 to select the controlchannels of the next group g+1 determined in step S5. In the currentcase, control channels of the second group are selected.

In such a way, when the process returns to step S2 to select next group,as in the aforementioned manner, the receiving level of each controlchannel in the selected group g is measured and it is checked if acontrol channel having a receiving level equal to or greater than thepredetermined value L₁ exists based on the received levels. The loopprocess of steps S2-S7 is repeated until a control channel having areceiving level equal to or greater than the first predetermined valueL₁ is detected. Incidentally, if g is G+1 in step S6, the variable g isreset to 1 in step S7 to return to the first group.

In step S3, if a control channel having a receiving level equal to orgreater than the first predetermined value L₁ is detected as the resultof the receiving level measurement mentioned above, it is judged thatthe mobile station is approaching to the service area. Then, the processproceeds to the second control channel selection mode SM2 described instep S8 and after.

In the first step S8 in the second control channel selection mode SM2,the receiving levels of all the N control channels CH1-CHN prestored inthe channel memory 13M of the mobile station are measured. Then, each ofthe measured levels of all the control channels is checked to see if anyone of the levels is equal to or greater than the second predeterminedvalue L₂ (step S9). If control channels having receiving levels equal toor greater than the second predetermined value L₂ are detected, each ofthese control channels is checked from the highest level control channelin descending order to see if any one of these control channels can bethe call waiting control channel (step S10). The predetermined value L₂is a reference value to be compared with to determine whether processingfor call-waiting can be performed or not with the selected controlchannel, and is selected to be equal to or greater than L₁, for exampleto 8-10 dBμ. The decision on propriety for the call waiting is performedby receiving the control signal on the selected control channel and thenby judging if the mobile station can receive and send a call over thecontrol channel. The method of this decision is the same as mentionedabove.

If it is decided that call waiting is possible in this decision ofpropriety for call waiting, a call waiting or a call sending isperformed on the control channel (step S11).

When any control channel having the receiving level equal to or greaterthan the second predetermined value L₂ is not detected in the step S9,or when all the detected control channels are not qualified for callwaiting in the decision of propriety for call waiting in step S10 evenif the control channels having the receiving levels equal to or greaterthan the second predetermined L₂ are detected, the process returns tothe first step S1 to move to the first control channel selection modeSM1 again and a forecast for the service area is performed.

Next, the effect of the embodiment performing the intermittentoperations in accordance with the first aspect will be explainedreferring to FIGS. 5A and 5B. FIG. 5A is a timing diagram showing theoperation of the conventional control channel searching method and FIG.5B is a timing diagram showing the intermitted operation of the controlchannel searching method in accordance with the first aspect of thepresent invention. Incidentally, in FIGS. 5A and 5B, N is the number ofall the control channels prestored in a mobile station, T_(OFF) is thetime duration where the power of the receiving circuit is off, n is thenumber of control channels per group (n=N/G), G is the number of groups(G=1 through N groups), t_(m) is time required for measuring thereceiving level per channel, I₁ is the consumed current value during thereceiving level measurement and I₀ is the consumed current value whenthe power of the receiving circuit is off.

As shown in FIG. 5A, in the conventional control channel searchingmethod, the control channels having receiving levels equal to or greaterthan the predetermined value are detected for every receiving levelmeasurement of all the control channels and then the decision ofpropriety for call waiting is performed on those detected controlchannels. Therefore, the power of the receiving circuit is always on.The average value I_(AV) of the consumed current until a call waitingcontrol channel is determined is I₁ mA. On the other hand, in thecontrol channel searching method of the present invention, since theoperation is intermittent in the first selection mode until the servicearea is detected as shown in FIG. 5B, if the time period of the secondselection mode SM2 is shorter enough to be ignored than the time periodof the first selection mode SM1, the average value I_(AV) of theconsumed current until a call waiting control channel is determined isexpressed as below.

    I.sub.AV ={(n×t.sub.m)×I.sub.1 +T.sub.OFF ×I.sub.0 }/(n×t.sub.m +T.sub.OFF)

In order to further clarify the effect of the present invention, if I₀=10 mA, I₁ =100 mA, T_(OFF) is 1 second, n=5 (N=50, G=10) and t_(m) =10ms are assumed, the average consumed current in the conventional methodis I_(AV) =100 mA and the average consumed current in the method of thepresent invention is I_(AV=) 18 mA. Thus, it is appreciated that theconsumed current in the method of the present invention is considerablyreduced.

In the embodiment mentioned above, although an example where a mobilestation is approaching to the service area, in a place where the controlchannel cannot be received, such as a tunnel, under ground and abuilding, the first control channel selection mode SM1 is performedrepeatedly under the control of the mobile station, and thus, the powersupply to the receiving circuit is periodically turned off for apredetermined time period T_(OFF) to reduce the consumed current.

Embodiment According To The Second Aspect

Next, an embodiment according to the second aspect of the presentinvention will be explained below. As described above, in the case wherea mobile station is entering the service area or a mobile station isexiting -.from a state incapable of receiving a control channel in theservice area, a call waiting control channel is newly selected from allthe control channels. In such a selection of a call waiting controlchannel, if interleave channels are used in the mobile communicationsystem, in this embodiment, the interleave channels are excluded fromthe control channels whose receiving levels are equal to or greater thanthe predetermined value L₁ selected from all the control channels. Thedecision of propriety for call waiting is performed only for theremaining control channels to reduce the number of measurements of thereceiving levels and to reduce the power consumption.

In this embodiment of the second aspect of the present invention, whensearching a call waiting control channel, a received signal on theselected control channel is checked to see if the selected controlchannel is interleave channel based on the frequency information of thecontrol channel and the receiving level. The control channel which isjudged as an interleave channel is not treated as a candidate controlchannel for call waiting and the propriety for call waiting is notdecided on the control channel. The process flow is shown in FIG. 6 andis explained below referring to the specific examples in FIGS. 7A-7D.However, the operation examples explained here are based on theconditions below.

(a) Total number of the control channels which can be preset in a mobilestation is 20 and those control channels are CH1-CH20.

(b) Assuming that the frequency of channel CHn is Fn and 6 is a positivevalue less than the channel frequency interval .increment.F, a frequencyinterval between the adjacent channels in these 20 channels is less thanthe channel frequency interval .increment.F as shown below. ##EQU1## (c)The predetermined value of the receiving level for allowing call waitingis 10 dbμ, for example.

First, in step S₁, the mobile station refers to the frequencyinformation of all the control channels CH1-CH20 prestored in thechannel memory of the mobile station for call waiting and measures thereceiving levels R of those control channels. Each receiving level R ofeach control channel obtained in this measurement is stored in the tableTB1 (FIG. 7A) provided in the receiving level memory 12 (FIG. 3).

Then in step S2, it is decided whether or not there are control channelswhose receiving levels are equal to or greater than the predeterminedvalue L₁ referring to each receiving level stored in the table TB1. Ifthere is no control channel having a receiving level equal to or greaterthan the predetermined value L₁, then the process returns to step S1 tostart the measurement of the receiving levels of all the controlchannels again.

In step S3, if it is decided in step S2 that there is at least onecontrol channel having a receiving level equal to or greater than thepredetermined value L₁, the receiving levels equal to or greater thanthe predetermined value L₁ are extracted from the receiving levels ofthe control channels stored in the table TB1 and sorted in descendingorder. Each set of a control channel number and its receiving levelobtained in such a way is stored in the candidate table TB2 (FIG. 7B)provided in the receiving level memory 12. In the candidate table TB2 ofFIG. 7B, the receiving levels R equal to or greater than thepredetermined value L₁ extracted from the table TB1 of FIG. 7A aresorted in descending order.

Based on the candidate table TB2 created in such a manner, therelationship between the frequencies of the control channels is analyzedstarting from the control channel of highest receiving level indescending order (i.e., starting from the highest receiving levelcontrol channel sequentially in the candidate table TB2). If theinterval between compared frequencies of the two control channels isless than the channel frequency interval .increment.F, the channelhaving lower receiving level is presumed as an interleave channel anddeleted from the candidate control channels. The details of the deletionprocess are as follows.

First, a variable m is set to 1 in step 4. Then, referring to the mth(=1st) storage area TB2(m) of the candidate table TB2 in step 5, thereceiving level R(m) of the control channel stored in the storage areaTB2(m) is read out to check to see if R(m)=0. If R(m)=0, then theprocess goes to step S14 skipping the processes S6-S13. If R(m) is notzero, the process proceeds to step S6. In the example of TB2 of FIG. 7B,the receiving level R(1)=50 of channel CH5 is read out and then theprocess proceeds to step S6.

In step S6, the control channel number CH(m) of the area TB2(m) is readout and the corresponding frequency F(m) is read out from the channelmemory 13M (FIG.3). In this example, the frequency F₅ of the channelnumber CH5 is read out.

Then, in step S7, a variable j=m+1 is set. In step S8, a receiving levelR(j) of the control channel of the area TB2(j) is read out to check tosee if R(j)=0. If R(j) is zero, the process immediately skips to stepS12. If R(j) is not zero, jth storage area TB2(j) in the candidate tableTB2 is referred to read out the channel number CH(j) stored in this areaTB2(j) and then the corresponding frequency F(j) is obtained from thechannel memory 13M. In step S10, the difference between the twofrequencies F(m) and F(j) is checked to see if the difference is lessthan the control channel interval .increment.F. If the difference isless than the control channel interval .increment.F, in step S11,receiving level R(j) of channel CH(j) is compulsorily set to the valueless than the predetermined value 10 dBt, e.g. 0 (refer to table TB2' ofFIG. 7C). If the difference is not less than .increment.F, the processmoves to step S12. By this process, the frequency difference |F5-F6|between the channel CH5 and the channel CH6 having next largest level inthe table TB2 is obtained. Since this difference is less than.increment.F from aforementioned conditions, the process proceeds tostep S11 and the receiving level R(6) of the channel CH6 is set to 0.

In step S12, j is checked to see if j is M. If j is not M, j isincremented by 1 in step S13 and the process returns to step S7 torepeat steps S8-S13 until. j becomes M. When j becomes M, in step S14, mis checked to see if m is M. If m is not M, m is incremented by 1 instep S15 and the process returns to step S5 to repeat steps S5-S15 untilm becomes M. By this process, for example, regarding the channel CH14,the receiving levels R(13) and R(15) of the respective channels CH13 andCH15 whose frequency intervals relative to the channel CH14 are lessthan .increment.F and whose receiving levels R(13) and R(15) are lessthan the channel CH14, are also set to 0. In such a way the table TB2'of FIG. 7C is obtained.

When m becomes M, in step S16, a table TB3 (FIG. 7D) is obtained bydeleting all the channels having receiving levels of zero (R=0) from thetable TB2'. In the table TB3, the data are also sorted in descendingorder starting from the highest receiving level. The number of controlchannels stored in the table TB3 at this time is K.

In step S17, a variable k is set to 1. In step S18, the channel numberCH(k) of the area TB3(k) of the table TB3 is read out and this controlchannel is set to the receiving circuit to perform the receivingoperation and to decide the propriety for call waiting. That is, it ischecked to see if the receiving level of the channel CH(k) is equal toor higher than the predetermined value, if the control signal cannormally be received, if the analysis of the control signal indicatesthe contents defining conditions for call waiting and the mobile stationcan satisfy the conditions for call waiting, and if the control signalis not of a control signal inhibiting the use of the control channel. Ifcall waiting is not possible, k is checked to see if k is M in step S19.If k is not M, k is incremented by 1 in step S20 and the process returnsto step S18 to repeat the same processes.

If the control channel set in step S18 is decided that the controlchannel can be a call waiting channel, the call waiting state isstarted. If k is K in step S19, that is, any one of the control channelsin the table TB3 is not qualified as a call waiting control channel, theprocess returns to step S1 to start the measurement of the receivinglevels of all the control channels again. In such a way, the candidatecontrol channels in the table TB3 of FIG. 7D are checked sequentially indescending order starting from the highest level control channel to seeif these are capable for call waiting.

As mentioned above, according to the embodiment of the second aspect ofthe present invention, the number of decisions of propriety for callwaiting can be reduced and the time up to the start of a call waitingcan be shortened. Also, the battery consumption in a mobile station canbe reduced by the reduced number of decisions of propriety for callwaiting. It is obvious that the method of this second aspect may also beapplied to the second control channel selection mode in theaforementioned method of the first aspect.

Embodiment According To The Third Aspect

In the embodiment of the third aspect of the present invention, thepower consumption is reduced by measuring the moving speed of the mobilestation, deciding if the moving speed is slower than a predeterminedvalue, and setting the longer measuring intervals for a plurality ofcontrol channels when the moving speed is slower than the predeterminedvalue compared with the case where the moving speed is faster. FIG. 8shows a configuration example of a mobile station for implementing themethod of the third aspect. The same reference numerals are assigned tothe portions corresponding to those in FIG. 3. As shown in FIG. 8, amoving speed detection part 11B and a speed memory 12B are additionallyprovided on the mobile station. A receiving level measuring part 11Aperforms a measurement of the receiving levels of the control channelsbased on each frequency set by the control part 13 when a call waitingcontrol channel is searched as explained in the first and second aspectsor performs periodic measurements of the receiving levels of the callwaiting control channel and the control channels of the peripheral cellswhen the mobile station is in the call waiting state. The measuredreceiving levels are stored in the receiving level memory 12A of thememory part 12 in correspondence to the respective channel numbers, asrequired basis. In a mobile station, the fading frequency of thereceiving level almost corresponds to the moving speed of the mobilestation. In this embodiment, the measured receiving level of the callwaiting channel is provided to the moving speed detection part 11B,where the fading frequency is detected at equal intervals, independentlyof the processing for dividing cell-switching, as a value correspondingto the moving speed. The detected fading frequency is written into thespeed memory 12B. If the fading frequency read out from the speed memory12B is less than a predetermined value, the control part 13 sets therepetition period of the measurement of receiving level longer.

In the speed determination process by the control part 13, the speed isdetermined as high speed when, for example, the fading frequency isequal to or higher than 20 Hz and is determined as low speed when thefading frequency is lower than 20 Hz. For specific method for detectinga fading frequency, as shown in the Japanese Patent Application laidopen No. 3-104330, for example, the amplitude envelope of the receivinglevel of the call waiting control channel is detected in the movingspeed detection part 11B. The amplitude envelope is converted to abinary value based on whether or not the amplitude envelope is equal toor greater than a predetermined value. The number of pulses of thebinary output within a predetermined time period is counted and thecounted value is the value corresponding to the fading frequency, i.e.,the value corresponding to the moving speed of the mobile station. Thiscounted value is written into the speed memory 12B. If the fadingfrequency is equal to or greater than the predetermined value, thecontrol part 13 determines that the moving speed of the mobile stationis high. If the fading frequency is less than the predetermined value,the moving speed of the mobile station is determined as low.

Next, the receiving level measuring process and the cell switchingdetermination process which are the important portions of the method ofthe third aspect of the present invention will be explained referring toFIG. 9.

First, in step S1, time T₁ is set to a first timer t₁. In step S2, thefirst timer t₁ is started and then in step S3, measurement of thereceiving level of the call waiting control channel and measurement ofeach receiving level of the control channel of each peripheral cellspecified by the information in the call waiting control channel arestarted. In step S4, the measured receiving level is accumulated in thememory area corresponding to the measured channel in the receiving levelmemory 12A. Also, the receiving level of the call waiting channel isgiven to the speed detection part 11B. In step S5, a check is made tosee if all the measurements of the receiving levels of these controlchannels are completed. If completed, in step S6, the moving speed(fading frequency) stored in the speed memory 12B is compared with apredetermined value. If the moving speed is decided as high speed, theprocess immediately moves to step S7. If the moving speed is decided aslow speed, a second timer t₂ is started in step S8 while the receivinglevel measurement of the control channels is refrained. In step S9, whenthe second timer t₂ is up, the process moves to step S7. The preset timeT₁ of the first timer t₁ is, for example, 15 seconds and the preset time.increment.T of the second timer t₂ is, for example, approximately 50ms.

In step S7, a check is made to see if the first timer t₁ is up, and ifnot, the process returns to step S3 to measure the receiving level ofeach control channel again. If the first timer t₁ is up, the processmoves to step S10. In step S10, an average receiving level is obtainedby dividing the accumulated level value of each channel in the receivinglevel memory 12A by the number of accumulations. In step Sll, thecontrol part 13 in FIG. 8 compares the value of the average receivinglevel of the call waiting control channel plus a predetermined valuewith each of the average receiving levels of the control channels of theperipheral cells. If any one of the average receiving level of thecontrol channel of the peripheral cell is not greater than the averagereceiving level of the call waiting control channel plus thepredetermined value, the process returns to step S1 to repeat theaforementioned process. If any one of the average level of the controlchannel of the peripheral cell is greater than the average receivinglevel of the call waiting control channel plus the predetermined value,a cell switching process is performed in step S12 and the processreturns to step S1.

Since the process is performed in such a way, if the fading frequency ishigh, i.e., if the moving speed is high, as shown in FIG. 10A, when thereceiving level measurement for all the control channels required to bemeasured is completed, the receiving level measurement for these controlchannels are immediately started again. The time T_(M) required formeasuring the receiving levels once for all the control channels to bemeasured becomes a measurement repetition period T_(P). In such a way,the measurement of the receiving levels is continuously repeated andwhen the first timer t₁ is up, a check is performed to see if theprocessing for cell switching is necessary. Namely, the repetitionperiod T_(S) of the cell switching decision becomes T₁. This is the sameas the conventional repetition period of receiving level measurement andcell switching.

If the fading frequency is low, i.e., if the moving speed is slow, whenthe receiving level measurement for all the control channels required tobe measured is completed, the receiving level measurement is startedagain after the preset time .increment.T of the second timer t₂ is up.In this case, as shown in FIG. 10B, the receiving level measuring periodT_(P) is T_(P) =T_(M) +.increment.T. .increment.T is selected so thatthe measuring period T_(P) is shorter than the deciding period T_(S),desirably T_(P) <T_(S) /2. The deciding period T_(S) of the cellswitching is same as T₁ of FIG. 10A. In such a way, since the receivinglevel measurement is performed intermittently, the power consumptionrequired for the measurement is reduced. Although the number of measuredreceiving levels used in a cell switching deciding process is less thanthe case of FIG. 10A, the reliability is not much deteriorated becauseof the slow variation in speed of the receiving level.

However, low fading frequency does not necessarily mean small change inreceiving level change. If number of measurements is reduced forobtaining average values of the receiving levels in a constant timeperiod, the average receiving levels are easily influenced by a largeinstantaneous change in the receiving level and the possibility for thereduced accuracy becomes high. Therefore, highly reliable receivinglevel information can be obtained by the longer deciding period T_(S)for cell switching. This embodiment is shown in FIG. 11 and the samereference symbols are assigned to the steps corresponding to those inFIG. 9. Namely, in this embodiment, when the process for each measuringperiod T₁ is started, in step 01, the moving speed stored in the speedmemory 12B is checked to see if the speed is high or low. If high speed,in step S1, time T₁ is preset to the first timer t₁ and the processmoves to step S2. If low speed, in step S02, a measuring period T₂ whichis longer than T₁ is preset to the first timer t₁ and the process movesto step S2. The other processes are identical to those in the case ofFIG. 9. For example, T₁ is approximately 15 seconds and T₂ isapproximately 3 minutes.

Therefore, if the fading frequency is high and the moving speed is high,the process is the same as the case of FIG. 9. In this case, as shown inFIG. 10A, the measuring period is T_(P) =T_(M) and the cell switchingdecision period is T_(S) =T₁. However, if the fading frequency is lowand the moving speed is slow, T₂ is set to the first timer t₁.Therefore, as shown in FIG. 10C, the cell switching decision periodT_(S) is T₂, and thus the period is longer than the high speed case.Therefore, more receiving levels can be used for the cell switchingdecision, the average receiving level is sufficiently represented evenin the longer fading period, highly reliable receiving level informationcan be obtained, and thus the cell switching decision can accurately beperformed accordingly.

In the above process, the fading frequency is categorized into twolevels "high" or "low", and corresponding to these two levels, themeasuring period T_(P) and the cell switching decision period T_(S) arechanged in two levels, respectively. The fading frequency can also becategorized into three or more levels. In such a case, the measuringperiod T_(P) and cell switching decision period T_(S) may be changed inthree or more levels, respectively, in correspondence in three or morefading frequency levels. In addition, instead of detecting the fadingfrequency, the moving speed of the mobile station can be directlymeasured to change the T_(P) and T_(S) in response to the speed. In thecase of a car telephone, the speed can be detected by the speed detectorinstalled on the car. In the case of a portable telephone, a speeddetector may be installed in the portable telephone. In the embodimentsshown in FIGS. 9 and 11, an average value of the receiving levels foreach control channel is obtained in every time period T_(S). However,decision of propriety for call waiting can be done using a runningaverage of the predetermined number of receiving levels in thepredetermined time interval longer than T_(S) for every period T_(S)going back to the past from that point in time.

In FIG. 9, the period T₁ for deciding propriety for call waiting isdetermined by setting the time T₁ to the timer t₁ in step S1. However,for example, when a series of the receiving level measurements in stepsS3-S5 is repeated predetermined number of times for the call waitingcontrol channel and the control channels of the peripheral cells, it maybe assumed that time T₁ is up. In this case, a predetermined repeatnumber H is set as t₁ in step S2. At immediately before step S6 or atthe immediately before step S7, t, is decremented by 1 and when t₁ is 0in step S7, it is decided that time T₁ is up. In the similar manner, inthe embodiment of FIG. 11, the time periods T₁ and T₂ for decidingpropriety for call waiting may be defined respectively by thepredetermined repeat numbers H1 and H2 of the receiving levelmeasurements and the completion of the period T₁ or T₂ may be decided bycounting the number of repeat times of the measurements.

As described above, in the embodiment of the third aspect of the presentinvention, the moving speed of a mobile station is detected. If themoving speed is slower than a predetermined value, non-measuring timeperiod is increased to reduce the power consumption by intermittentlymeasuring the receiving levels of the peripheral control channels. Inthis case, highly reliable receiving level information can be obtainedby making the cell switching decision period longer, and the cellswitching decision and the cell switching control can be performedaccurately.

We claim:
 1. A control channel searching method which a mobile stationin call waiting state performs, wherein a power supply for a receivingcircuit is turned off during a break time of receiving levelmeasurements, comprising steps of:(a) measuring the receiving levels ofa call waiting control channel and control channels of peripheral cellsfor every first time period; (b) obtaining a speed value correspondingto a moving speed of said mobile station; (c) checking to see if saidspeed value is smaller than a predetermined constant value; (d)inserting a break time for receiving level measurements by changing saidfirst time period to a time period longer than that when said speedvalue is smaller than said constant value; and (e) performing cellswitching decision for every said second time period based on themeasured receiving levels by checking to see if the receiving level ofthe control channel of the peripheral cell is predetermined value higherthan the receiving level of said call waiting control channel.
 2. Themethod according to claim 1 wherein said second time period is longer inthe case where said speed value is less than said predetermined valuecompared with the case where said speed value is equal to or greaterthan said predetermined value.
 3. The method according to claim 2wherein said step (b) detects the fading frequency of the receivinglevel of said call waiting control channel as said speed value.
 4. Themethod according to claim 1 or 2 wherein said step (b) detects themoving speed of said mobile station as said speed value.
 5. The methodaccording to claim 1 or 2 wherein the cell switching decision of saidstep (e) is performed based on the average receiving level of thereceiving levels measured in every said first time period of said step(a) repeated within said second time period.
 6. The method according toclaim 1 or 2 wherein the cell switching decision of said step (e) isperformed based on the running average of said receiving levels by saidstep (a) in a predetermined time period longer than said second timeperiod going back to the past.
 7. The method according to claim 1wherein said second time period is a predetermined time length.
 8. Themethod according to claim 1 wherein said second time period is apredetermined number of repeat times of the measurement of said step(a).